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Spacetime: A Relativity of Time

The cold winds of November signal that the end of the year is coming. Looking back on the year, Kingos may feel that time goes fast. Then again, one doubt may come to Kingos’ mind all of a sudden: whether the speed of time is constant or relative. The answer to this doubt is in Einstein’s thesis the Theory of Relativity, which has brought tremendous changes to the concept of time and space since it was published. The thesis discloses the interdependent relationship between time and space. This month, the Sungkyun Times (SKT) introduces the concept of spacetime, explains its usage in human society, and examines spacetime in the everyday life of Kingos.

What Is Spacetime?

Time and Space Conceived by Humans

People conceive of time and space separately. They think that time always flows, while space stays still. For example, a girl made an appointment with her boyfriend three days ago at Gangnam Station. But unexpectedly, the appointment was postponed to today. She thinks the appointment itself changed in time, but not in space. Space, however, always changes. Space means not only location, but every object that exists in that space. That is to say, everything in Gangnam station changes in three days: cracks on the ground, newly opened stores, the weather, and even her boyfriend’s feelings toward her.

This ever-changing space has a close relationship with time. To illustrate, if there is no change in space, there is no time. If everything in Gangnam Station today has been perfectly identical for three days, we cannot say that “three days have passed.” Time exists only when space changes. This interdependent relationship of time and space created a new word “spacetime,” which stands for the integration of time and space.

Spacetime in Science

Modern physicists have found out that speed of time is relative depending on two factors: speed of space and the gravity that a space gets.

①Speed of Space

One experiment in Einstein’s thesis proves that the speed of space changes with time. Imagine that there is a train running at a high speed. Inside the train, a person throws an apple up in the air and then catches it back. Everyone on the train may think the movement of the apple moves vertically up and down. To people outside the train, however, the trace of the apple draws a parabola, whose length is much longer than the vertical one. This means that outside the train, the apple moves as if it were caught in slow motion. At this point, replace an apple with a light. The moving distance of the light is still longer when observed outside the train. According to the formula to calculate distance, speed, and time in physics, speed equals the distance traveled divided by time. Surprisingly, the speed of the light is consistent everywhere in the universe. Therefore, since the distance that the light traveled is longer, time also becomes longer inside the fast running train.

Time flows slowly inside the fast running train.

This phenomenon was proven to be true in the 1970s by muon particles, one of the cosmic rays shooting toward the earth from space. The surviving time of muon particles was only two millionths of a second, so muon particles had to disappear in the atmosphere. Nevertheless, in reality, muon particles were observed on the earth’s surface. The speed of muon particles is so fast that it approximates to the speed of light. This causes time in muon particles to go slower. As a result, the surviving time of muon particles increased by more than 100 times.

②Strength of Gravity

If someone places a heavy rock on a bed mattress, the surface of the bed mattress will curve. When another small sized object is placed on the mattress, the object will roll over the curve, getting nearer and nearer to the heavy rock. This rollover phenomenon is called gravity. The curve on the mattress surface refers to the curvature of spacetime. That is, near the heavy object, spacetime curves and causes other objects to be dragged toward the heavy object.

How a Star is Observed in the Wrong Location Because of the Sun’s Gravity

In fact, gravity not only drags the object but also makes time go slower. As proof, the stars near the sun are observed in distorted locations. Distortion is caused by gravity since the light is bent around the massive sun, and the original starting point of the light is recognized with distortion. Near the sun, the sun drags the light and causes distortion in its pathway; the distance that light travels increases. The speed of light, however, is the same everywhere in the universe. Therefore, according to the formula to calculate distance, speed, and time, the time passed increases near the sun. As a result, this phenomenon shows that time goes slower near a massive object.

Spacetime Used in Human Society

Usage in Technology: Global Positioning System (GPS)

GPS Satellites Orbiting the Earth

The location information that people usually use is provided by GPS satellites orbiting the earth. To be specific, GPS satellites report where a smartphone is located right now and helps people get to their destination when using a map application. The location information that GPS satellites provide is necessary for the navigation of cars, ships, and airplanes because subtle errors can cause devastating results.

The time of GPS satellites, however, is actually slower than that of the earth by about 0.0004 seconds per day. This is due to two factors: the fast speed of GPS satellites and the different strengths of gravity they experience. First, GPS satellites move at a speed of more than 10 kilometers per hour. This makes time inside the satellites go slightly slower than on the earth. Second, due to their far distances away from the earth, GPS satellites experience less gravitational force than on the earth’s surface. The gravity in GPS satellites is four times weaker than that on the earth’s surface. This weak gravitational force makes time inside the satellites go a little faster. As a result, these factors cause a discordance between the two times. By understanding the concept of time and space scientifically, the error in technology can be removed.

Usage on Art: Cubism

Introduction of spacetime caused artists to realize the fact that time is a movement of space. Accordingly, many artists began to put the element of “time” onto the canvas. The element allowed them to depict the world more accurately. Cubism is the most representative example of this. Cubism is an art period of the early 20th century, which depicted changing objects along with time rather than time-stopped objects. For example, Marcel Duchamp used Cubism in his work named Nude Descending a Staircase. Before Cubism, a painting was just a snapshot of a particular moment in which the element of time did not exist. In Duchamp’s paintings, however, several continuous snapshots of spacetime were simultaneously depicted on one screen, which made the flow of spacetime exist in the painting.

Hommage a Marcel Duchamp by Gjon Mili(left), Nude Decending a Staircase by Marcel Duchamp (right)

Spacetime in Everyday Life That Humans Did Not Realize

Inaccurate Age System That Humans Use

Humans measure their age with an internationally unified watch. But there is a gap between the age humans count and the real biologically recognized age. Many studies show that humans get older more quickly if they live at a higher altitude. Every time altitude increases by 30 centimeters, humans get older by one billionth of a second. As an example, the time on Mount Everest flows faster by one 3500th of a second than on the earth’s surface. The far distance from a massive object, the earth, lessens the gravitational force, which makes time go faster. Humans count their age uniformly no matter where they live. The time measured by humans, however, does not really measure the actual flow of time.

Integration of Time and Space

①Difference Between Future Value and Present Value of Good

All situations surrounding humans always change. People do not know what will happen at the very next moment. As everything in the future can be changed, people often transfer some present value into future value. Insurance products for certain circumstances such as car accidents or sudden deaths are examples of this. Paying money periodically, consumers expect help for future circumstances.

Bank deposits also preserve present value into future value. When present money is spent in the future, however, it does not have the same value because the situation surrounding the money has changed a lot. To solve this problem, interest is added to match the difference in values. Therefore, a bank’s task is to transfer present value into future value exactly. This transformation process includes making appropriate interest rates to minimize the difference between future value and present value.

② Preserving Spacetime Through Photographs and Videos

Humans are mortal creatures who cannot avoid death along with time. So they have tried to preserve a series of events they experienced through art. The first attempt of this can be seen in Paleolithic cave paintings in the Lascaux cave, which depicted the scenes of successful hunts. By painting a series of successful hunting events, humans aimed to capture valuable spacetime and furthermore hoped these events would last forever. In Western art, the better the painting preserved spacetime, the more the painting was considered artistic. Along with the invention of photography, capturing the moment of spacetime became possible, which achieved the ultimate goal of Western art.

The Lascaux Caves Wall Paintings of the Paleolithic

In the past, only a few educated artists could have the privilege of preserving time and space. With the popularization of photo and video technology, however, the public has also been able to record each of its valuable spacetime. Furthermore, as video technology evolved, not only snapshots of spacetime but also spacetime itself became possible to be recorded. Nowadays, people share the records of their valuable spacetime on social media such as Instagram.

Kingos may have their own speed of time. Yet, firmly setting their own destinations and deadlines, they often push themselves faster than their own speed. Therefore, it is important for them to remember that the flows of time are relative. Time flows at a different speed person by person, and it is necessary to live up to a Kingo’s own speed of time.

이소윤  oyatleee@skku.edu

<저작권자 © THE SUNGKYUN TIMES, 무단 전재 및 재배포 금지>

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